1. Field of the Invention
The present invention relates to a semiconductor device such as a thin film transistor (TFT), a method of fabricating the same, a display device such as a liquid crystal display (LCD), and a method of fabricating the same.
2. Description of the Background Art
A thin film transistor (hereinafter referred to as a polycrystalline silicon TFT) employing a polycrystalline silicon film which is formed on a transparent insulating substrate as an active layer is recently being developed as each pixel driving element (pixel driving transistor) for an active matrix LCD.
The polycrystalline silicon TFT advantageously has larger mobility and higher drivability as compared with a thin film transistor employing an amorphous silicon film as an active layer. When such polycrystalline silicon TFTs are employed, therefore, an LCD of high performance LCD can be implemented while not only a pixel part (display part) but a peripheral driving circuit (driver part) can be integrally formed on the same substrate.
In such a polycrystalline silicon TFT, the polycrystalline silicon film for serving as an active layer can be formed by a method of directly depositing the polycrystalline silicon film on the substrate, a method of forming an amorphous silicon film on the substrate and thereafter polycrystallizing the same, or the like.
The method of directly depositing the polycrystalline silicon film on the substrate has a relatively simple step of depositing the film by CVD, for example, under a high temperature, for example.
On the other hand, the amorphous silicon film which is deposited on the substrate is thereafter polycrystallized by solid-phase crystallization in general. This solid-phase crystallization is adapted to polycrystallize the amorphous silicon film in a solid state by performing a heat treatment, for obtaining the polycrystalline silicon film.
An example of such solid-phase crystallization is now described with reference to FIGS. 31 and 32.
Step A (see FIG. 31): An amorphous silicon film is formed on an insulating substrate 51 of quartz glass, for example, by general low pressure CVD, and a heat treatment is performed in a nitrogen (N2) atmosphere at a temperature of about 900xc2x0 C., thereby solid-phase growing the amorphous silicon film and forming a polycrystalline silicon film 52.
The polycrystalline silicon film 52 is worked into a prescribed shape by photolithography and dry etching by RIE, to be employed as an active layer of a thin film transistor.
A silicon oxide film for serving as a gate insulating film 53 is deposited on the polycrystalline silicon film 52 by low pressure CVD.
Step B (see FIG. 32): A polycrystalline silicon film 55 is deposited on the gate insulating film 53 by low pressure CVD, an impurity is implanted into this polycrystalline silicon film, and a heat treatment is performed for activating the impurity.
Then, a silicon oxide film 54 is deposited on the polycrystalline silicon film by normal pressure CVD, and thereafter the polycrystalline silicon film and the silicon oxide film 54 are worked into prescribed shapes by photolithography and dry etching by RIE. The polycrystalline silicon film is employed as a gate electrode 55.
Then, an impurity is implanted into the polycrystalline silicon film 52 by self alignment through the gate electrode 55 and the silicon oxide film 54 serving as masks, for forming source/drain regions 56.
This method is called a high temperature process since high temperatures of about 900xc2x0 C. are employed for the solid-phase crystallization and the impurity activation, and has such an advantage that the treatment time can be shortened when a substrate such as a quartz substrate, for example, having a high insulation property is employed.
However, such a substrate having a high insulation property is high-priced, while a relatively low-priced glass substrate unpreferably causes heat distortion. In recent years, therefore, development is generally made in a low temperature process which allows the employment of the glass substrate.
In particular, improvement of performance is indispensable in a TFT which is a driving device. Therefore, various attempts have been made in order to improve the quality of the material forming the TFT and the like through the low temperature process.
For example, a technique of forming a polycrystalline silicon thin film, for example, by excimer laser annealing with a starting material of an amorphous silicon film has been developed as a technique of improving the quality of an active layer material influencing the device characteristics.
However, the laser annealing disadvantageously requires a long time for the crystallization process, since a beam operation must be repeatedly performed. In case of employing only a laser beam as a heat source, the laser annealing requiring a long time must also be performed for activating an impurity region, for example, in addition to the polycrystallization process, and hence the total process time is increased to reduce the throughput of such a TFT device or an LCD device employing the TFT.
An object of the present invention is to enable a low temperature process with employment of a low-priced substrate, for reducing the cost for fabricating a thin film transistor or a liquid crystal display.
Another object of the present invention is to improve the throughput in fabrication of a thin film transistor or a liquid crystal display by fabricating a high-quality polycrystalline silicon film in a short time.
Still another object of the present invention is to fabricate a semiconductor device having excellent quality with a homogeneously activated impurity region.
A further object of the present invention is to fabricate a semiconductor device having a high-quality semiconductor film in a short time.
A further object of the present invention is to provide a display device such as an LCD device having excellent display performance.
A further object of the present invention is to prevent deformation of a substrate in a heat treatment.
A further object of the present invention is to prevent warp and breakage of a substrate in case of employing RTA (rapid thermal annealing) as a heat treatment.
A method of fabricating a thin film transistor according to a first aspect of the present invention is adapted to set the temperature for a heat treatment for crystallizing an active layer which is formed on a substrate is set at a level not deforming the substrate, for example 600-700xc2x0 C., for activating an impurity by a heat treatment method which is different from that employed for this heat treatment.
According to the first aspect of the present invention, polycrystallization of an amorphous silicon film and activation of an impurity region can be performed by properly combining the heat treatment method employing a temperature not deforming the substrate, laser annealing and RTA with each other, whereby the fabrication time is shorted as compared with a method of performing both of polycrystallization and activation by laser annealing.
According to a preferred embodiment of the first aspect, the method comprises the steps of forming an amorphous silicon film on an insulating substrate, heat treating the amorphous silicon film by laser annealing or RTA (rapid thermal annealing) employing a temperature not deforming the substrate thereby forming a polycrystalline silicon film, forming a gate electrode on the polycrystalline silicon film with an intervening gate insulating film, forming an impurity region in the polycrystalline silicon film, and activating the impurity region by rapid heating employing RTA or laser annealing.
According to this method, a number of substrates can be simultaneously treated in solid-phase crystallization.
In the first aspect of the present invention, the amorphous silicon film may contain microcrystals. When such an amorphous silicon film containing microcrystals is polycrystallized by solid-phase crystallization, the crystal growth can be completed in a short time.
In the first aspect, the gate electrode may have at. least the amorphous silicon film, and may be crystallized by the heat treatment for activating the impurity. In this method, crystallization of the amorphous silicon film and activation of the impurity are performed at once, whereby the treatment time is shortened as compared with a method of performing these operations independently of each other.
In the first aspect, the gate electrode may have a two-layer structure of at least a silicon film and a metal or metal silicide film, and may be reduced in resistance by the heat treatment for activating the impurity. According to this method, reduction of resistance of the two-layer structure of the silicon film and the metal or metal silicide film and activation of the impurity are performed at once, whereby the treatment time is shorted as compared with the case of separately performing these operations.
The gate electrode comprising the two-layer structure of a silicon film and a metal or metal silicide film may be provided so that reduction in resistance of the gate electrode and activation of the impurity region are simultaneously performed by RTA or laser annealing.
In the first aspect, light irradiation heat from a lamp may be employed as a heat source for the RTA. This lamp may be formed by an xenon arc lamp. A heat treatment which is more suitable for activation of the impurity can be performed by employing such a lamp.
A thin film transistor fabricated by the method of fabricating a thin film transistor according the first aspect of the present invention can be employed as each pixel driving element of a liquid crystal display. Alternatively, the thin film transistor can be employed as each peripheral driving circuit element of the liquid crystal display. Thus, an excellent liquid crystal display can be fabricated in a short time.
A semiconductor device according to a second aspect of the present invention comprises a heat absorption film which is formed on a substrate, a semiconductor film which is formed on the heat absorption film, a gate electrode which is formed on the semiconductor film through a gate insulating film, and an impurity region which is formed in the semiconductor film, and the heat absorption film is provided within a region substantially corresponding to the semiconductor film.
According to the second aspect, a semiconductor device having an impurity region of a homogeneously activated state can be obtained due to the presence of the heat absorption film.
In the second aspect, an insulating film may be provided between the heat absorption film and the semiconductor film.
In the second aspect, the heat absorption film may be provided in a size and within a region substantially corresponding to a channel region in the semiconductor film. Thus, the function of the heat absorption film properly acts on a necessary portion, so that no bad influence is exerted on the remaining portion such as the substrate, for example.
In the second aspect, the heat absorption film may be made of a conductive material such as a metal or metal silicide, or a semiconductor material such as silicon. Thus, the device can be electrostatically shielded against ions which are present in the substrate since the heat absorption film is made of a conductive or semiconductor material.
In the second aspect, the heat absorption film may have a shading property. When the semiconductor device is employed for a display device such as an LCD, the quantity of light directly entering the semiconductor device can be reduced due to the shading property of the heat absorption film.
In the second aspect, the substrate may be a transparent substrate.
The semiconductor device according to the second aspect of the present invention can be employed as at least one of each pixel driving element or each peripheral driving circuit element of a liquid crystal display. Thus, an excellent display device can be obtained.
A method of fabricating a semiconductor device according to a third aspect of the present invention is adapted to provide a semiconductor film for serving as an active layer of a transistor on a substrate through a heat absorption film, and to activate an impurity region provided in the semiconductor film by a heat treatment.
According to the fabrication method of the third aspect, a semiconductor device having an impurity region which is in an excellently and homogeneously activated state can be obtained.
According to a preferred embodiment of the present invention, the method comprises the steps of forming a heat absorption film on a transparent substrate, forming a semiconductor film on the heat absorption film, forming a gate electrode on the semiconductor film through a gate insulating film, forming an impurity region in the semiconductor film, and activating the impurity region by a heat treatment, and the heat absorption film being provided within a region substantially corresponding to the semiconductor film.
According to another preferred embodiment of the present invention, the method comprises the steps of forming a heat absorption film on a transparent substrate, working the heat absorption film into a prescribed shape, covering the heat absorption film with an insulating film, forming a semiconductor film for serving as an active layer of a transistor on the insulating film, forming a gate electrode on the semiconductor film through a gate insulating film, forming an impurity region in the semiconductor film, and activating the impurity region by a heat treatment, and the heat absorption film being provided within a region substantially corresponding to the semiconductor film.
In the third aspect, the semiconductor film may be prepared by polycrystallizing an amorphous silicon film by a heat treatment.
In the third aspect, the heat treatment may be performed by laser annealing.
In the third aspect, the heat absorption film may be made of a conductive material such as a metal or metal silicide, or a semiconductor material such as silicon. The device can be electrostatically shielded against ions which are present in the substrate by preparing the heat absorption film from a conductive or semiconductor material.
The heat absorption film may have a shading property. Thus, the quantity of light directly entering the semiconductor device can be reduced when the semiconductor device is applied to a display device such as an LCD.
RTA may be employed as the heat treatment. In this case, the impurity can be activated in a short time without influencing the substrate.
The heat source for RTA can be formed by a xenon arc lamp. In this case, heat absorption can be efficiently performed.
A semiconductor device fabricated by the method according to the third aspect of the present invention can be employed as at least one of each pixel driving element and each peripheral driving circuit element of a liquid crystal display. Thus, an excellent display device can be fabricated in a short time.
A semiconductor device according to a fourth aspect of the present invention comprises a plurality of semiconductor elements which are integrated on a substrate. Heat absorption films are provided between the substrate and the semiconductor elements, and an area or film thickness of each heat absorption film is relatively reduced in a portion where a relatively large number of the semiconductor elements are provided, while an area or film thickness of each heat absorption film is relatively increased in a portion where a relatively small number of the semiconductor elements are provided, in accordance with the distributed state of the semiconductor elements on the substrate.
According to a preferred embodiment of the fourth aspect, a plurality of semiconductor switching elements are integrated on a substrate, and each semiconductor switching element comprises a heat absorption film which is formed on the substrate, a semiconductor film which is formed on the heat absorption film, a gate electrode which is formed on the semiconductor film through a gate insulating film, and an impurity region which is formed in the semiconductor film. An area or film thickness of each heat absorption film is relatively reduced in a portion where a relatively large number of the semiconductor switching elements are provided, while an area or film thickness of each heat absorption film is relatively increased in a portion where a relatively small number of the semiconductor switching elements are provided, in accordance with the distributed state of the semiconductor switching elements on the substrate.
In the fourth aspect, the heat absorption effects of the heat absorption films can be adjusted by changing the areas and/or the thicknesses of the heat absorption films.
In the fourth aspect, the heat absorption effects of the heat absorption films can be adjusted by providing semiconductor switching elements having no heat absorption films and changing the ratio of presence of such semiconductor switching elements.
Each of the heat absorption films in the fourth aspect can be prepared from a film which is similar to those according to the second and third aspects.
A display device according to the fourth aspect of the present invention is a driver-integrated display device comprising a pixel part and a peripheral driving circuit part which are formed on the same substrate. This display device comprises pixel driving elements which are provided in the pixel part and peripheral driving circuit elements which are provided in the peripheral driving circuit part, and the pixel driving elements and the peripheral driving circuit elements are formed by semiconductor switching elements. Each semiconductor switching element comprises a heat absorption film which is formed on the substrate, a semiconductor film which is formed on the heat absorption film, a gate electrode which is formed on the semiconductor film through a gate insulating film, and an impurity region which is formed on the semiconductor film. A ratio of area or film thickness of the heat absorption film relative to the semiconductor film in the pixel part is adjusted to be larger than that of the heat absorption film in the peripheral driving circuit part.
The pixel part and the peripheral driving circuit part can be provided on one of a pair of substrates which are opposed to each other through a liquid crystal layer. Each of the heat absorption films can be formed by a film which is similar to those of the second and third aspects.
The heat absorption effects of the heat absorption films can be adjusted by changing the areas or thicknesses of the heat absorption films.
Alternatively, the heat absorption effects of the heat absorption films can be adjusted by providing semiconductor switching elements having no heat absorption films and changing the ratio of presence of these semiconductor switching elements.
A method of fabricating a semiconductor device according to a fifth aspect of the present invention employs RTA for a heat treatment in a process of forming a semiconductor element on a substrate, and heating by RTA is performed in a plurality of times, while the heating temperature is increased stepwise from the initial time toward the final time.
According to a preferred embodiment of the fifth aspect, the fabrication method comprises the steps of forming a semiconductor film on a substrate, forming a gate electrode on the semiconductor film through a gate insulating film, forming an impurity region in the semiconductor film, and activating the impurity region by a heat treatment through RTA, while heating by RTA is performed a plurality of times and the heating temperature is increased stepwise from the initial time toward the final time.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.